Metal oxide primary cells are generally constructed from a pressed metal oxide (e.g., manganese oxide, MnO2) cathode, an anode prepared from zinc powder and an alkaline electrolyte usually disposed within a separator. All of these components are contained within an appropriate casing or housing. Usually, the current from the cathode is collected from the core by a metallic member such as a portion of the cell steel casing or a metal cathode holder (e.g., a cup or clip) which holds the pressed cathode mass. Constructions of some cells of this type are disclosed in U.S. Pat. Nos. 3,939,010, 3,945,847, 4,125,638, and 4,585,715.
After prolonged storage, the discharge rate of metal oxide cells is substantially reduced below that of freshly prepared cells. This is primarily caused by formation of a poorly conducting oxidized layer on the surface of the current collector. Since the manganese dioxide is in direct contact with the current collector, the metal on the surface of the collector (e.g., nickel) is oxidized forming a layer of metal oxide on the surface of the collector. The presence of such a layer increases the internal resistance of the cell with a concomitant decrease in cell performance.
Others have attempted to solve the collector oxidation problem by employing collector materials which were as inert as possible. In U.S. Pat. No. 2,861,116, platinum, palladium, tantalum, molybdenum, silver, nickel, lead, gold, titanium, zirconium and carbon were used as collector materials. German Pat. No. 1,421,582 found that gold was a preferred (although expensive) collector material. U.S. Pat. No. 4,760,002 teaches the coating of a nickel plated steel collector with a layer of cobalt.
One disadvantage of most of these systems is that they fail to take into account and properly balance considerations of the nobility, expense and solubility of the coating material. For example, use of gold has been found to be ineffective because, although it is a good noble metal, it was very expensive and had a tendency to redissolve in cells causing shorts. In other applications, "degree" of nobility is sacrificed to save expense, usually with a concomitant reduction in immediate and/or longterm cell performance. The present invention provides a cathode current collector which can be manufactured to reduce formation of a resistive oxide layer on the collector surface thus providing increased immediate cell performance without making substantial sacrifices with respect to expense or overall longterm cell performance.